/usr/bin/psd-overlay-helper is in profile-sync-daemon 6.31-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43  | #!/bin/bash
readonly PATH=/usr/bin:/bin
readonly IFS=$' \t\n'
while getopts :v:l:u:w:d: OPT; do
	case $OPT in
		v|+v)
			declare -ir OLFSVER=$OPTARG
			;;
		l|+l)
			readonly BACKUP=$OPTARG
			;;
		u|+u)
			readonly UPPER=$OPTARG
			;;
		w|+w)
			readonly WORK=$OPTARG
			;;
		d|+d)
			readonly TMP=$OPTARG
			;;
	esac
done
shift $(( OPTIND - 1 ))
OPTIND=1
## TODO - pass error codes back to psd so it can break if the mount command fails
case "$1" in
	mountup)
		if [[ $OLFSVER -eq 23 ]]; then
			mount -o nosuid,nodev -t overlay overlaid -olowerdir="$BACKUP",upperdir="$UPPER",workdir="$WORK" "$TMP"
		elif [[ $OLFSVER -eq 22 ]]; then
			mount -o nosuid,nodev -t overlayfs overlaid -olowerdir="$BACKUP",upperdir="$UPPER" "$TMP"
		fi
		;;
	mountdown)
		umount -l "$TMP"
		;;
	*)
		echo "Do not call this script directly; psd will do so for you. Thank you, come again."
		exit 0
		;;
esac
 |